The invention relates to an acoustic actuator plate structure comprising an electret layer containing electret material and a support structure layer.
Electret plate structures include plastic films with a permanent or long-lasting electric charge in their plastic material. These electret films are used in acoustic applications, such as microphones and loudspeakers.
The material of electret films may be conventional plastics, such as polycarbonate PC, polyester or polypropene PP. However, the charge of such electret films diminishes quite quickly due to moisture or temperature, for instance.
In polar plastic materials it is possible that the charge field visible outside the film may not only diminish but also change its sign to the opposite, whereby the electric properties of applications do not remain stable even though the charge level remains. The electret properties of electret films made of fluoropolymers, such as polytetrafluoroethylene PTFE, keep quite well even in high temperatures and humid conditions. However, fluoropolymers are quite expensive and their processing is quite difficult, which limits their use in electret films.
Publication U.S. Pat. No. 4,626,263 discloses an electret that contains non-polar polymer and polar polymer and modified non-polar polymer. All the above-mentioned substances are mixed and the mixture is extruded into film, for example. However, the manufacturing process of such an electret is all in all quite complex. In addition, the properties of the electret are not altogether sufficiently good in particular for the most demanding acoustic applications.
Cyclic olefin copolymer COC is well suited as material for electret films, because it keeps its charge quite well and its price is also reasonable. For practical applications of electret films, essential properties also include that cyclic olefin copolymer COC can be metalized easily without pre-treatment to form electrically conductive surfaces and that water penetrates the material poorly. Films made of cyclic olefin copolymers that keep their charge well and endure high temperatures are, however, quite brittle, which limits the use of the material both in manufacturing film and in practical applications.
Publication U.S. Pat. No. 6,489,033 discloses a solution which aims at improving the durability and machinability of cyclic olefin copolymer COC by forming a mixture of cyclic olefin copolymer COC and some other different polyolefin. The mixture is then formed into an electret film. However, doping cyclic olefin copolymer COC or adding additives thereto to reduce the brittleness impairs its ability to keep its charge in practical conditions. Thus, the disclosed solution cannot produce a sufficiently good electret film in particular for the most demanding acoustic applications.
Publication GB 1,394,347 discloses an electret film for use in loudspeakers, earphones or, as biased film, in microphones. This electret film is a two-layer element with polytetrafluoroethylene PTFE or fluoroethylenepropene FEP, for instance, as the electret material. Their yield point is so low that alone they are not suitable for the intended use. Therefore, the publication proposes the use of a support layer with a higher yield point than that of the electret material. The density of materials used as electret materials and having a low yield point is quite high. Thus, the structure has some mass-induced inertia and the resonance frequency of the structure will be quite low. All in all, the energy requirement is quite high especially when producing high frequencies.